This invention relates to the art of chemically treating an aluminum surface. More specifically, it relates to the art of treating an aluminum surface to improve both the corrosion resistance and the adhesion of an organic finish subsequently applied to the treated surface. This invention also concerns a process for accomplishing the foregoing results with an aqueous solution which has a less detrimental effect upon the environment than conventional treating solutions because it does not require the presence of chromium or phosphate in the solution.
This invention provides an even further advantage in the manufacture and preparation of aluminum cans. The concentration and processing conditions can be adjusted if desired so that the aluminum surface is transformed without the formation of any measurable coating on the surface. Typically, no organic finish is applied to the bottom of aluminum cans. The absence of any substantial coating permits the manufacturer to improve the corrosion resistance of those unfinished portions without changing the appearance of the surface of the aluminum.
In the processing of aluminum cans after forming, the following procedure is typical:
1. WASH WITH WARM WATER;
2. CLEAN, USUALLY WITH AN ACID-TYPE CLEANER;
3. WATER RINSE;
4. APPLY TREATMENT CHEMICAL;
5. WATER RINSE;
6. DEIONIZED WATER RINSE;
7. DRY;
8. APPLY DECORATIVE ORGANIC FINISH TO THE CAN EXTERIOR;
9. CURE ORGANIC FINISH AT ELEVATED TEMPERATURE;
10. APPLY INTERIOR SANITARY LACQUER; AND
11. CURE INTERIOR SANITARY LACQUER.
For step (4), standard practice is to employ an aqueous solution containing from 1/2 to 1 weight percent of a mixture of hexavalent chromium, phosphoric acid, and fluoride. Such treating solutions have produced satisfactory quality in terms of both corrosion resistance and paint adhesion. However, the chromium and phosphate components are environmentally objectionable, and their use therefore entails additional recovery or waste treatment expense. It would, therefore, be highly desirable to be able to use a treating solution which would produce acceptable results which did not at the same time create the environmental problem of solutions containing chromium and phosphate.
The use of tannins in connection with metal treating has been suggested by the prior art. U.S. Pat. No. 2,502,441 discloses an alkali metal phosphatizing solution containing a two-component accelerator which may be used for the treatment of iron and steel surfaces and also possibly for other metals such as aluminum. The accelerator portion of the composition contains either a molybdenum or tungsten compound and a phenolic substance such as a tannin. The patentee notes, however, that if the alkali metal phosphate plus tannin is used without the molybdenum compound, deposition of a coating seems to be completely inhibited. U.S. Pat. No. 2,854,368 teaches the use of a phosphoric acid solution containing a tannin for the treatment of iron or steel and also possibly for other metals such as aluminum. The most dilute solution suggested by the patentee is one containing one mole of phosphoric acid per liter and ond weight percent tannin. When this solution was substituted for that of the present invention, completely unsatisfactory organic finish adhesions were obtained.